The present invention relates generally to control methods and control devices for controlling compressor stations, and more particularly to the methods and apparatuses for controlling parallel and series operated dynamic compressors.
All known control systems for parallel working compressors and for compressors working in series can be divided into two categories. In the first category, the antisurge protective devices and the control device for controlling the station gas parameter are independent and not connected at all to each other. The station control device changes the performances of individual compressors by establishing the preset gains and biases which remain constant during station operation. For some compressors, the gains are equal to zero and the biases are set to provide for a baseload operation, with a constant and often maximum speed. This category of control system can not cope with two major problems.
The first problem is associated with the necessity to vary the gains and biases for load sharing device set-points, for optimum load-sharing under changes of station operating conditions, such as inlet conditions or deterioration of some machines. The second problem is associated with possible interactions between the station control device and the antisurge control devices of individual compressors under conditions when the process flow demand is continuously decreasing. It is very usual for this category of control system to operate one compressor far from surge while keeping one or more compressors dangerously close to surge, including premature antisurge flow to prevent surge.
In the second control system category, there is a cascade combination of the station control device and the load-sharing devices of individual machines. In this category, the station control device manipulates the set points for the distances between the individual operating points and the respective surge limits.
If, for the parallel operation, some stabilization means is effective to make such cascade approach workable, then for series operation it will not work at all. But even for parallel operation, the above identified stabilization means degrades the dynamic precision of control.
To overcome the aforementioned problems, the dynamic control of compressors may be significantly improved for both parallel and series operated machines by eliminating cascading but still providing for equalization of relative distances to the respective surge control lines. It can be even further improved by providing special interconnection between all control loops to eliminate dangerous interactions in the vicinity of surge.